194 



THE EYE IN EVOLUTION 



organs are probably purely tactile, and between these two regions the sense organs 

 are compound since they contain both visual and tactile cells (Fig. 89).^ 



Chsetognath, 

 Sagitta 



Subsidiary Invertebrate Phyla 



For convenience, four small and subsidiary phyla of the Inverte- 

 brates are most usefully considered here. 



CH^TOGNATHA (" bristle-jawed ") or arrow-worms, delicate, translucent 

 torpedo -shaped creatures comprising some 30 species which swim in incredible 

 numbers in great shoals among the plankton of all seas, have well-developed 

 eyes. Spadella, for example, or Sagitta, has two composite simple eyes at the 

 anterior extremity of its body, formed by the union of 5 ocelli, the structure of 

 which has already been described (Fig. 132) ; although presumably tripartite, 

 the nerve fibre from each eye is gathered into a single optic nerve trunk. 



Figs. 180 and 181. — The Eyes of Rotifera, 



L 



Fig. 180. — The cerebral eye. 



Section through the cerebral gan- 

 glion of Synchceta, showing two cere- 

 bral eyes, E (after Peters). 



Fig. 181.— The frontal 

 eye. 



The eye of Rhinoglena 

 with pigment spot, P, and 

 refractile lens, L (after 

 Stossberg). 



Rotifer 



Bryozoa 



BOTiFERA (" wheel-bearers "), the beautiful minute wheel-animalcules, 

 sometimes of fantastic shape, which swim so abvindantly with the aid of a crown 

 of cilia like revolving wheels in fresh water, damp moss or the sea all the 

 world over, are usually highly light-sensitive. There is a generalized dermatoptic 

 sense which evokes a positive phototaxis, but exact orientation is determined 

 by the eyes and varies with their morphological development (Viaud, 1938-43). 

 Frequently there is a single or paired cerebral eye embedded in the dorsal 

 nerve ganglion {Synchceta) (Fig. 180). In other species, sometimes in addition 

 to the cerebral eyes, there is one or two frontal or lateral eyes (Fig. 181). The 

 cerebral eye consists of a single cell resembling a brain cell ; the lateral or 

 frontal eyes are epidermal cells inside which is a lens-like body associated with 

 a mass of red pigment (Peters, 1931 ; Stossberg, 1932). Branchionus, one of 

 the commonest members of this class which inhabits ponds and ditches in 

 abundance, has a simple unpaired eye surrounded by red pigment and associated 

 with tufts of sensory hairs, situated where the cerebral ganglion comes into 

 contact with the body-wall just behind the wheel of cilia at the anterior end 

 of the animal. 



POLYZOA (bryozoa), very ancient plant-like organisms which include fresh- 

 W:ter and marine forms (sea-mats, etc.) are sessile colonial corallines or " moss 

 ari Is " which grow in tufts on the shores or in pools all over the world encrust- 



1 p. 133. 



